1. Technical Field of the Invention
The invention disclosed and claimed herein generally pertains to an apparatus for use in connection with a quadrature switching mixer configured to reject an image signal associated with a received RF signal. More particularly, the invention pertains to an apparatus of the above type which substantially reduces leakage between the in phase (I) and quadrature (Q) output branches of the mixer, and thereby improves the conversion gain of the mixer and avoids deterioration of the mixer output signal.
2. History of the Related Art
In radio equipment, the signals transmitted through the air occupy one frequency band, whereas the signals that are processed occupy a considerably lower frequency band. Accordingly, a mixer is used to translate or convert the radio frequency signals to an intermediate frequency (IF). The mixing process, or heterodyning, is multiplicative, that is, the input signal is multiplied by a local oscillator signal (in the time domain). As a result of the multiplication, however, the output of the mixer may include contributions from the desired signal as well an undesired image signal. Those of ordinary skill in the radio reception art know that the image signal is a signal whose frequency is capable of being converted, via the mixing process, to the same IF as the desired signal.
Image rejection mixers have been developed which use the principle of canceling to reduce the contribution of the image signal at the mixer output. In one commonly used type of image rejection mixer, the desired input signal (RF) is split into two signal components: an in-phase (I) component (RFI) and a quadrature (Q) component (RFQ). The quadrature (Q) signal is delayed 90 degrees relative to the in-phase (I) signal, that is: RFQ(ωt)=RFI(ωt−π/2). The local oscillator signal is also split into a quadrature signal LOQ which is delayed 90 degrees relative to the in-phase oscillator signal LOI. Mathematically, in complex notation, the image rejection mixer works as a multiplication of the input signal RFI+jRFQ with the local oscillator signal LOI+jLOQ.
As described hereinafter in further detail, the multiplication is usually implemented with commuting switches. Each commuting switch has a pair of complementary switches SW and {overscore (SW)}. When the SW switch is closed, the output signal of the commuting switch has the same polarity as the input signal, and when the {overscore (SW )} switch is closed, the output signal of the commuting switch has a different polarity than the input signal. Summing junctions are then provided for cancellation of the components representing the undesired image signal. This type of image rejection mixer is referred to as a quadrature switching mixer.
The components for a quadrature switching mixer of the type described above can be fabricated using any suitable semiconductor technology such as CMOS, BJT, and the like. This provides certain important advantages when the mixers are used in wireless receivers for small portable electronic devices, such as mobile phones and the like for use in UMTS (Universal Mobile Telecommunications System), Bluetooth, and other wireless communication systems. CMOS based quadrature switching mixers, however, have a serious drawback in that the I and Q output signal components of the mixer, OUTI and OUTQ respectively, tend to short together. That is, a portion of one of the quadrature branch outputs may flow backward through one of the switches in the mixer, and enter the input of the other branch. This leakage can cause a conventional quadrature image rejection mixer of the type mentioned above to become less useful in practice, because of the resulting large conversion loss. Moreover, the leakage between the two outputs OUTI and OUTQ deteriorates the cumulative mixer output signal and leads to poor image rejection ratios.
One presently used approach to reduce the leakage between the I and Q output signals of a quadrature switching mixer is to add resistors in series with the switches, thereby effectively isolating the I and Q branches from each other. A switch, however, should have low resistance when it is closed to minimize any signal loss. Adding isolation resistors introduces an additional loss. This loss can be significant, even though it is usually less than the loss resulting from leakage between the I and Q branch output signals. The added resistors also cause an undesirable voltage drop, especially if the signal is in current mode, which means that the input impedance of the mixer should be kept low. Therefore, it is undesirable to add any further resistors to the quadrature switching mixer.
Accordingly, it would be desirable to be able to reduce the leakage between the I and Q output signals of a quadrature switching mixer, and to be able to do so without introducing additional loss to the mixer such as from isolation resistors.